Recently, as an inorganic photoconductive material, amorphous silicon, amorphous selenium, cadmium sulfide, zinc oxide, etc., are used, but some of these materials are expensive owing to the difficulty of the production thereof and some of them cause a problem from the view point of environmental protection owing to the toxicity of them.
On the other hand, as an organic photoconductive material, in particular, a function separating type light-sensitive material comprising a combination of a charge generating material and a charge transporting material has been positively proposed (e.g., U.S. Pat. No. 3,791,826). In the system, by using a material showing a high carrier generation efficiency as the charge generating material and combining the charge generating material and a material having a high charge transportability as a charge transporting material, there is a possibility of obtaining an electrophotographic photoreceptor having a high sensitivity.
In these materials, the charge transporting material is required to efficiently receive carriers generated in the charge generating material by the irradiation of light under the application of an electric field, quickly transport the carriers in the photoreceptor layer, and quickly erase the surface carrier.
The transferring velocity of a carrier per unit electric field is called carrier drift mobility. A high carrier drift mobility means that the carrier transfers quickly in the charge transporting layer.
The carrier drift mobility is specific to the charge transporting material, and hence in order to attain the high carrier drift mobility, it is necessary to use a material showing a high carrier drift mobility. The carrier drift mobility of conventional materials has not yet reached a sufficient level at present.
On the other hand, since carrier drift mobility depends upon the concentration of the charge transporting material, a method of increasing the concentration of a charge transporting material in a charge transporting layer is employed. The case that the concentration of a charge transporting material becomes the highest is the case that the charge transporting layer is formed by the charge transporting material only and such a charge transporting layer is formed by a vapor deposition method, etc. For example, an organic electroluminescence (EL) device, etc., is prepared by the method as described above [e.g., C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 51, 913(1987)].
However, in the case of forming such a charge transporting layer containing a charge transporting material at a high concentration, there are problems of the deposition of crystals and the formation of pin holes, whereby it is difficult to form a uniform layer.
Also, even when the characteristics of both the charge generating material and the charge transporting material are good, it is important that the injection of carriers from the charge generating material into the charge transporting material, that is, the injection of electrostatic charges from the charge generating layer into the charge transporting layer, be carried out with good efficiency. The injection of the carrier depends upon the characteristics of the interface between a charge generating material (or a charge generating layer) and a charge transporting material (or a charge transporting layer) and hence there is not identity between the kinds of the materials being used. As described above, various conditions are required for a charge transporting material.
Hitherto, as a charge transporting material, for example, a distyryl compound represented by the following formula (II) is proposed in JP-A-63-269158 (the term "JP-A" as used herein means an "unexamined published Japanese patent application": ##STR2## wherein Ar.sub.1 to Ar.sub.4 each independently represents an alkyl group, an aralkyl group, an aromatic hydrocarbon group which may have a substituent, or an aromatic heterocyclic group which may have a substituent and A represents an alkyl group, an aralkyl group, an aryl group or a heterocyclic group, wherein each group may have a substituent.
Also, JP-A-1-284858 discloses a styryl compound represented by the following formula (III): ##STR3## wherein Ar.sub.1 represents an alkyl group, an aralkyl group, an aryl group which may have a substituent, or a heterocyclic group which may have a substituent; Ar.sub.4 represents a hydrogen atom, an alkyl group, an aralkyl group, or an aryl group which may have a substituent; Ar.sub.2 and Ar.sub.5 each represents an aryl group which may have a substituent, an aromatic polycyclic group, or a heterocyclic group; Ar.sub.3 represents an arylene group which may have a substituent or a divalent heterocyclic group which may have a substituent; n represents 1 or 2; and said Ar.sub.1 and Ar.sub.2 and said Ar.sub.4 and Ar.sub.5 may combine with each other to form a ring.
Also, 1,4-bis(4,4-diphenyl-1,3-butadienyl)benzene having a structure near the styryl compound described above is described in Chem. Ber., 93, 1799-1809(1960), but the use of the compound for an electrophotographic photoreceptor is not stated. When the compound is intended for use as an electrophotographic photoreceptor, the solubility of the compound in a binder polymer is poor and hence the compound is hard to utilize.
As other compounds, for example, 1,4-bis[4-(p-dimethylaminophenyl)-1,3-butadienyl]benzene shown by the following formula ##STR4## and 1,4-bis[2-(p-dimethylaminophenyl)vinyl]benzene shown by the following formula ##STR5## are known but the solubility of these compounds in a binder polymer is also poor.
In the case of the conventional compounds as described above, when more than about 2 parts by weight of the compound is added to 1 part by weight of a binder polymer, there are problems that the solubility of the compound in the binder polymer is poor and even when the compound is dissolved in the binder polymer, in the case of forming a film or layer, the compound is crystallized, pin holes are formed, and the film or the layer is whitened or becomes brittle, and hence there is a restriction on the addition amount in the case of using the compound.
Thus, the development of a new material which is stable even when the addition amount is increased and can show a high carrier drift mobility has been desired.